Virtual protection channel for fiber optic ring network

ABSTRACT

A single, normally inactive, protection ring, provides protection against a working channel failure in a fiber optic ring network. The protection channel is established and put into operation by a series of steps that are triggered by the detection of signal degradation or loss of light from one transponder by the other transponder. Each node reacts to a failure of another node in substantially the same way. Incoming traffic from external sources is rerouted from a working input-output interface to a protection interface, to establish a path for the protection signal. Signals are sent between nodes enabling the two nodes to resume communication. Only the portion of the protection ring between nodes is placed into service.

The benefit of filing dates of Provisional Patent Applications Ser. No.60/293,232, filed May 25, 2001 and Ser. No. 60/293,233, filed May 25,2001 is hereby claimed.

FIELD OF THE INVENTION

The present invention relates to protection methods and circuits formulti-node fiber optic networks More particularly, the inventionpertains to a fiber optic ring network that provides a single, normallyinactive, optical protection channel for multiple optical communicationschannels.

BACKGROUND OF THE INVENTION

Fiber optic ring system design involves a balance between the need toprovide protection for multiple channels of communication, the desire tomaximize the bandwidth available for the communications function of thenetwork, and the costs of constructing and maintaining the network. In asingle-fiber ring where bandwidth considerations are secondary, aseparate protection channel can be reserved for each communicationschannel. Such arrangements, while effective and readily implemented,often have unacceptable high overhead at the expense of revenuegenerating traffic.

In some systems, signals are sent in both the clockwise and thecounterclockwise directions. The protection function takes advantage ofthe fact that a given signal can reach its destination via two distinctpaths.

Many known wavelength division multiplexing (WDM) fiber optictransmission systems are deployed for communications between two endnodes. In this configuration, to protect against optical transponderfailures, some systems use a one-by-one protection scheme.

In a one-by-one protection scheme, each working transponder has its owndedicated protection transponder. Some other systems use a one-by-Nprotection scheme, where one protection transponder serves to protect Nworking transponders. The same kind of protection schemes can be used ina ring configuration as are used for transporting multiple wavelengths,in the point-to-point configuration. As a result, in some known systems,required protection channels carried over a single fiber are equal tothe number of two-node communications on the ring.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a fiber optic ring communications network;

FIG. 2 is a diagram illustrating a prior art protection system for afiber optic ring communications network;

FIG. 3 is a diagram illustrating a fiber optic ring communicationsnetwork protection system for the network of FIG. 1;

FIG. 4 is a block diagram of exemplary establishment of a protectionchannel in a fiber optic ring network according to an embodiment of thecurrent invention;

FIG. 4A is a block diagram illustrating another example of establishingof a protection channel in a fiber optic ring network according to anembodiment of the current invention;

FIG. 5 is a diagram illustrating paths of communication between twonodes of the ring network of FIG. 1;

FIG. 5A is a diagram of an alternate configuration to that of FIG. 5;

FIG. 6 is a block diagram illustrating one exemplary method respondingto a failure at a node to enable a pair of transponders to automaticallyreestablish communications via a protection channel, pursuant to anembodiment of the current invention;

FIGS. 7A, B taken together are a block diagram illustrating anothermethod of responding to a failure of one transponder to enable a pair oftransponders to automatically reestablish communications via aprotection channel, pursuant to an embodiment of the current invention;and

FIG. 8 is a block diagram illustrating an alternate embodiment of theinvention.

DETAILED DESCRIPTION OF EMBODIMENTS

While this invention is susceptible of embodiment in many differentforms, there are shown in the drawing and will be described herein indetail specific embodiments thereof with the understanding that thepresent disclosure is to be considered as an exemplification of theprinciples of the invention and is not intended to limit the inventionto the specific embodiments illustrated.

The following describes a process, system and modules wherein anacceptable level of protection is provided, while maximizing the amountof bandwidth available for communications channels.

In the disclosed system, a single normally inactive protection channelprovides protection against failures at a node. Such failures include,without limitation, malfunctioning or failure of one of the transpondersat a node, degraded signals or unacceptable traffic bit error rates. Theprotection channel is established and put into service by a series ofsteps that are triggered by the failure detected at a receiving node.

A standardized transponder module is provided with functionality thatenables a pair of identical transponder modules at spaced apart nodes tojointly respond to the failure by establishing the protection channelbetween the two nodes with a minimal loss of data. More specifically,each node reacts to a loss of light from another node or other detectedfailure in the same way: by rerouting incoming traffic from externalsources from a working input-output interface (“IOB”) to a protectionIOB.

A plurality of switches establishes a path for the protection signal, byturning off a respective channel laser on the working IOB, and bysending a protection signal. The failure thus initiates a sequence ofsteps at the detecting node that culminates with the working IOB laserbeing turned off. This in turn, in one embodiment provides a loss oflight signal at the failed node that initiates the sequence of stepsthat will complete the protection channel path between the two nodes.The two nodes can then resume communications.

As depicted in FIG. 1, a WDM-based fiber optic communications ringnetwork 101 includes a plurality of nodes 103 (labeled A, B, C, and D)joined by fiber optic links 107 a, b, c, d. Each node receivescommunications signals from external (upstream) communications devices105 for transmission on the network, and receives communications signalsthrough the links of the network for transmission to such communicationdevices.

The signals are transmitted through the fiber optic ring network asoptical signals having different wavelengths, or “lambdas” usingwave-division multiplexing (WDM). A given fiber optic cable is capableof simultaneously carrying a plurality of lambdas. As is known, voice ordata, can be transmitted by modulating light transmitted at theserespective wavelengths.

Communication between two nodes will usually occur via one or morechannels, using one or more lambdas. A channel facilitates two-waycommunications between devices associated with two different nodes.Thus, for each channel, light for a predetermined wavelength, lambda, istransmitted between two nodes. When a signal is provided to a node by anexternal communications device, that signal is then used to modulate thebeam for transmission between the two nodes.

A known prior art system 10′ is illustrated in FIG. 2. In system 10′,pairs of nodes are connected by a fiber, such as fibers 107 b′, c′, d′,which carries communications channels 203 as well as protection channel205. Accordingly, each communication lambda has a protection lambdaassociated with it. The protection lambda will be used to carry therespective communications in the event of a failure of the respectivecommunication channel.

The number of protection channels in the prior art example of FIG. 2depends, at least in part, on the number of groups of channels whichhave the same starting and ending nodes, such as 10A′, B′; 10A′, C′ and10B′, D′. For example, at least three protection channels of differentwavelength would be needed in the configuration of FIG. 2.

FIG. 3 illustrates a portion of the system 20 which incorporates theprotection system of the current invention. In system 20, only onewavelength, a single protection channel, serves as protection for allexisting channel.

As illustrated in FIG. 3 a single optical channel 22 is assigned as aprotection channel for the entire ring, including nodes 20A, B. C, D. Solong as there are no failures, for example so long as all of the systemtransponders (for example, light producing lasers) function properly theprotection channel is not activated. When a transponder fails, forexample at node 20C, the protection channel 22 would be activated torestore traffic transport between nodes 20C, D. Other failures wouldalso cause the protection channel to be placed into service.

In the protection scheme of FIG. 3, only one channel need be dedicatedfor a protection function per optical add/drop multiplexed (ADM) ring.Span “A”, link 22C. Illustrated in FIG. 3 requires only one protectionchannel, not two as in FIG. 2. Additionally, the scheme of FIG. 3 doesaway with a need to manage multiple protection channels as in FIG. 2.

FIG. 4 illustrates an exemplary failure, a transponder failure at node20A, relating to a given channel at that node of system 20. A series ofsteps is then initiated that result in the protection channel beingautomatically established between two nodes such as 20A, C that haveexperienced a breakdown of a primary communications channel.

It will be understood that the invention is not limited to recoveringfrom a transponder failure. It can advantageously respond for example toa degraded signal or erroneous bit rates.

Structures and methods disclosed herein, as will be understood by thoseof skill in the art, are especially advantageous in that only oneprotection ring and one protection transponder per node are necessary:

-   -   1) irrespective of the number of nodes;    -   2) irrespective of the number of working channels; and    -   3) irrespective of the destination nodes of the signals being        carried by the working channels.

In addition, only one protection detector/transponder interface (PIOB)is required per node irrespective of the number of working pass-throughchannels and the number of detector/transponder add/drop interfaces(WIOBS) at any node.

FIG. 4A illustrates an exemplary failure at node 20B. The protectionchannel members 22 c, d are activated between nodes 20B, D to carrytraffic.

FIG. 5 provides a more detailed view of a pair of nodes 507, 509, forexample in system 10. The two nodes are in communication with eachother, via optical link 510, part of a fiber optic ring network 10 thatembodies the present invention. Nodes 507, 509 are substantiallyidentical and contain one or more working input-output interfaces(“WIOBs”) 527, 527-1 . . . 527-n and 531, 531-1 . . . 531-n and aprotection input-output interface (“PIOB”) 529 and 533 as the interfacesbetween the communications network and the external communicationsdevices.

Each node 507, 509 includes control circuits 507-1, 509-1. Thesecircuits interact with other node components as discussed subsequently.Circuits 507-1 and 590-1 receive the respective incoming supervisorychannel, such as 559, and transmit on the outgoing supervisory channel559′.

A WIOB for any given node includes a receiver or detector of a receivedlambda, and, a transponder such as a laser for generating a signalcorresponding to the wavelength of light representing a given channel.In the depicted embodiment, each node has a plurality of WIOBs and onePIOB, each having one laser. Each WIOB and PIOB is associated with aspecific channel.

The lasers though modulated operate substantially continuously, duringselected time intervals so that—absent any failure—nodes incommunication with each other over a channel receive signalssubstantially continuously from each other. When a transponder at agiven node fails and stops sending its signal, the channel between twonodes no longer can carry traffic. Alternately, other types of failuresalso disrupt traffic.

To continue carrying traffic, a channel for a protection signal must beimmediately enabled and placed in service. The current inventionaccomplishes this with circuitry to carry out a sequence of steps at agiven node upon detection of a failure, for example, loss of light froma node with which it is communicating. Two nodes that have experienced abreakdown communication between them can rapidly reestablishcommunication by activating the respective protection channel.

In normal operation, a signal 510 a from another node enters node 507 attraffic lambda drop 511. If node 507 is the receiving node for thecommunication on the signal, the traffic lambda drop 511 passes thesignal to the appropriate Working Input/Output Interface (WIOB) 527.This interface performs an opto/electric conversion and sends the signalout of the network toward its final destination, via switch 551 to acommunications device 553 external to the ring network.

If node 507 is not the receiving node for the signal, the traffic lambdadrop 511 couples the signal through Switch A to combiner 515. Combiner515 couples the signal to multiplexer 517, which then couples acomposite signal, via link 510 onto node 509.

If node 509 is the final in-network destination of the signal, thesignal is dropped at traffic lambda drop 519, coupled to respective WIOB531. The signal is converted and sent via switch 555 along to device 557outside the network.

If the signal is destined for a different node, node 509 couples italong in the same manner that node 507 coupled it along, and so forth,until it reaches its destination. Processes and circuitry forimplementing the functionality described above, such as causing signalsto be dropped at particular nodes (as illustrated by traffic lambdadrops 511 and 519) are well known to those with ordinary skill in theart and need not be described further.

In one embodiment of the invention, normal communication between nodes507 and 509 occurs as follows. Signals originating at an externalcommunication device, such as device 553, associated with node 507travel through switch 551 to WIOB 527 through multiplexer 517 and on tonode 509. Where node 509 is the receiving node for the signal, the thesignal is routed through WIOB 531, via switch 555 to externalcommunication device 557.

In the unidirectional system described herein, signals to be added fromdevice 557 travel through switch 555 and are transmitted by WIOB 531,through multiplexer 525, through any intervening nodes, then on to node507. At node 507, the signals to be dropped go through traffic lambdadrop 511, through WIOB 527, and on to external communication device 553.Thus, communication is established between external communicationdevices 553 and 557, with the communications signals flowing in the samedirection around the fiber optic ring network. In normal operation, theprotection channel carries no optical signal, and thus no protectionsignal is present in the network.

For exemplary purposes, consider node 507 the “failed end”—i.e., thenode where the failure occurs—and node 509 the “failure detection end.”FIG. 6 is a flow diagram of one method of implementing an operatingprotection channel. FIGS. 7A, B taken together are a flow diagram ofanother method of implementing an operating protection channel. Theprocess of FIG. 6 will be discussed first.

As illustrated in FIG. 6, with reference to FIG. 5 for exemplarypurposes only, the triggering event for the sequence of events leadingto establishment of the protection channel condition 610 is detection byWIOB 531, step 611, of failure detection end 509, for example, of a lossof light on the channel that had hitherto been received from failed end507. In a preferred embodiment, when the loss of light has lasted for acertain period of time, step 612 (10 usec.) the the protection sequenceis initiated. It will be understood that other failures will beresponded to similarly.

In the embodiment depicted in FIG. 6, the exemplary period of time is 10microseconds. It will also be understood that the invention can bepracticed with time periods ranging from 1 microsecond to thousands ofmicroseconds. As those of skill in the art will understand, the user ofthe invention needs to balance the need to be certain a loss of lighthas occurred (suggesting a longer period) against the need to avoidexcessive loss of data while the protection sequence is implemented(suggesting a shorter period).

Returning to FIG. 5, communications traffic originating at externalcommunications device 557 associated with failure detection end 509 isrerouted from WIOB 531 to PIOB 533 by operation of switch 555. At thispoint, in this embodiment, PIOB 533 is placed in the active mode, butnot in the “in-service” mode. In this condition, PIOB 533 emits adata-less signal.

Because Switches A, B 521 and 547 are still in their normal settings,the signal from PIOB 533 is not sent into the network. Thus, in theactive mode, PIOB 533 outputs a data-less signal on the protectionchannel wavelength, but, in FIG. 5, that signal is routed by switch 547to an isolater.

During this time, communications from the external communications device557 may be stored in memory for communication once the protection pathis established, or, may simply be lost. Depending on the type of data,the loss may or may not be significant. Higher level protocols may beused, as are known in the art, to recover from data losses.

As those of skill in the art will understand, the steps of reroutingexternal traffic to the protection interface PIOB and activating thatinterface IOB in the “active” but not “in-service” mode can, but neednot, occur substantially simultaneously. The protection channel isestablished by turning off the local laser on WIOB 531 and by changingthe settings of switch 547 and switch 521. Alternate processingarrangements come within the spirit and scope of the invention.

The new switching arrangement provides a path for the data-lessprotection signal coming from PIOB 533, through switch 547 tomultiplexer 525, and through the ring network to its final destinationat the failed end 507. Also, as noted above, step 615, the laserassociated with WIOB 531 is switched off. Switching off this laser notonly turns off a component that is no longer needed (since the channelassociated with that laser will be abandoned in favor of the protectionchannel), but also provides a signal back to the failed end 507, in theform of a loss of light from the failure detection end 509.

As just mentioned, WIOB 527 at the failed end detects a loss of lightfrom the failure detection end. As yet, the switches in failed end 507are not configured to permit the protection signal to pass through toPIOB 529. In the depicted embodiment, failed end WIOB 527 issubstantially identical in operation to failure detection end WIOB 531,and thus responds to the loss of light in the same way.

Upon detection of loss of light for the specified period (10microseconds in the depicted embodiment), step 619, communicationstraffic originating at external communications devices 553 associatedwith failed end 507 is rerouted from WIOB 527 to PIOB 529 by operationof switch 551. At this point, PIOB 529 is placed in the active mode, butnot in the “in-service” mode. Thus, PIOB 529 outputs a data-less signalon the protection channel wavelength. In FIG. 5, that signal is routedby switch 539 to an isolater, step 623.

WIOB 527 at the failed end turns off its laser (which may have alreadybeen turned off as a result of the failure) for the failed channel. Inaddition, switches 539 and 513 are switched to establish the protectionchannel path within node 507.

The new switching configuration in node 507 provides a path for theprotection signal coming into node 507 from node 509 to go through theTraffic Lambda Drop 511, through switch 513 to Lambda Protection Drop537 and on to PIOB 529. This switching thus completes the path for theprotection signal that originated at failure detection end 509, to thePIOB of failed end 507. Receipt of that protection signal causes PIOB529 to switch to the in-service mode, and thus to begin transmittingdata received from external devices 553 over the network, step 627.

The completion of the protection signal path from node 507 to node 509by the switching of switch 539 has resulted in a protection signal beingsent from PIOB 529 to PIOB 533.

IOB 533 has switched to the in-service mode, and thus can transmit datareceived from external devices 557 over the network. Thus, through theabove-described series of steps, a a protection signal channel has beenestablished between failed end 507 and failure detection end 509, andthe PIOBs at both ends have been switched on to the in-service mode.This permits a resumption of communications between the two nodes, suchthat data that was originally send along the failed wavelength is nowbeing transmitted along the protection wavelength. FIG. 5A illustratesan alternate node configuration for carring out the method of FIG.6.

FIG. 6 is a flow diagram that illustrates added details as to how twonodes that have experienced a communication breakdown can reestablish acommunication between themselves automatically, over a protectionchannel. The left side of FIG. 6 depicts events that occur at the failednode, and the right side depicts events that occur at the failuredetection node. Comparison of the two sides shows that each side reactedsubstantially the same way to a detected failure.

In the event of a transponder failure or other type of signaldegradation at the failure detection end, the failure detection end 509senses a loss of light or other symptoms of failure. Upon sensing thefailure symptomology for an exemplary 10 microseconds, the failuredetection end checks whether the virtual protection channel is alreadyin use in the network. If it is, then the protection sequence is notinitiated. If the protection channel has not been used, the protectionsequence is initiated.

In an alternate embodiment, using the supervisory channel 559, 559′, thecontrol circuits, such as 509-1, can determine which portion of theprotection channel is in service. If there is no overlap between thesegment of the protection channel that is in service and the segmentwhich now needs to be placed into service, both segments can be inservice simultaneously.

By way of example, the failure detection end, node 509, determineswhether a protection channel is in use or not in use based on a signal(or lack of a signal) from supervisory channel 559, depicted in FIG. 5.As those of skill will understand, supervisory channel 559 would be incommunication with all the nodes of the network, and receives and sendsinformation to and from the nodes regarding whether the protectionchannel is in use. Upon initiation of the protection sequence,supervisory channel 559 changes its state to indicate that theprotection channel is in use. Thus, when the protection channel isalready in use, the supervisory channel operates to prevent a secondinitiation of the protection sequence.

Hence, if a failure has been detected, for the exemplary 10microseconds, and the protection channel is not in use, the failuredetection end switches 555 and reroutes external traffic from device 557to its protection IOB, step 612 and activates the protection IOB in the“active” but not “in-service” mode step 613.

When the failure has persisted for the exemplary 20 microseconds, thefailure detection end turns off its local laser and switches, Switch A,521 the appropriate switch to establish the protection signal pathwithin that node, step 615. The action of turning off the local laser atthe failure detection end results in the failed end detecting a loss oflight from the failure detection end step 617.

Upon sensing the loss of light for 10 microseconds, the failed endreroutes external traffic via switch 551 to its protection IOB step 619and activates the protection IOB 529 in the “active” but not“in-service” mode. When the loss of light has persisted for 20microseconds, the the failed end turns off its local laser and switchesthe appropriate switches to establish the protection signal path withinthat node step 623.

Establishment of the protection signal path at both nodes, while bothPIOBs are in the active mode, results in a data-less protection signalbeing received by each PIOB from the other PIOB steps 623, 625. Uponreceipt of the protection signal, each PIOB switches to the in-servicemode steps 627, 629, thereby enabling the resumption of communicationbetween the two nodes.

Those of skill in the art will understand that a variety of steps couldbe implemented to carry out the above process without departing from thespirit and scope of the present invention.

FIG. 7A, B taken together are a flow diagram that illustrates how twonodes that have experienced a communication breakdown, reestablishcommunication between themselves automatically, using the protectionchannel. The left side of FIGS. 7A, B depict events that occur at thefailed node. The right side depicts events that occur at the failuredetection node.

In the event of a failure at the failed end, condition 710, the failuredetection end 509 senses the failure, step 712. Upon sensing the failurefor an exemplary 10 microseconds, step 714, the failure detection endchecks whether the virtual protection channel is already in use in thenetwork, step 716. If it is, then the protection sequence is terminated.If the protection channel has not been used, the protection sequencecontinues. Here too, in an alternate embodiment, two different,non-overlapping sections for the protection channel can be placed intoservice.

By way of example, the failure detection end, node 509, determineswhether a protection channel is in use or not in use based on a signal(or lack of a signal) from incoming supervisory channel 559, depicted inFIG. 5. As those of skill will understand, supervisory channel 559 wouldbe in communication with all the nodes of the network, and receives andsends information, output channel 559′, from and to the nodes regardingwhether the protection channel is in use. Upon initiation of theprotection sequence, supervisory channel 559 changes its state toindicate that the protection channel is in use.

Thus, when the protection channel is already in use, in the singlesegment embodiment of FIG. 7A, the supervisory channel 559 operates toprevent a second initiation of the protection sequence. In an alternateembodiment, not illustrated, using module control circuits, such as507-1, 509-1, a second, non-overlapping initiation of the protectionsequence, using a different portion of the protection ring, could beimplemented.

With reference to FIG. 7A, if the failure has been detected for theexemplary 10 microseconds, and the protection channel is not in use, instep 718, the transponder on the respective PIOB, such as PIOB 533 isactivated by control circuits 509-1 but not placed in service. In step720, the respective control circuitry, control circuits 509-1, transmitsa message on out-going supervisory channel 559′ to all other nodes innetwork 10 that the protection channel is being placed into service andinforming the failed node, node 507, of the failure.

In step 722, the state of switch A, switch 521, is changed. Controlcircuits 509-1, for example, in step 724, disable the electrical outputsignals from the respective WIOB, such as WIOB 531, to switch 555.Switch 555 is also activated to feed incoming signals to both WIOB 531and PIOB 533.

In step 730, control circuits at the failed node, circuits 507-1deactivate the transponder at that node, WIOB 527. In step 732, thetransponder on PIOB 529, is the respective PIOB, activate but not yetplaced in service. Element 551 is switched to direct incoming signalsfrom element 553 to PIOB 529 for transmission.

When PIOB 529 is ready, it so informs the respective control circuit,circuit 507-1, step 734. In response thereto, circuit 507-1 step 736,changes the state of switches A, B, switches 513, 539.

In step 738, the respective protection transponder on the PIOB, such asPIOB 529, is placed in service. An information carrying protectionsignal is coupled via switch B and multiplexer 517 to fiber 510.

The protection signal is received from fiber 510, from the failed node,node 507, condition 740. In step 742, the received protection signal,from the failed node, node 507, from PIOB 529 is coupled, via respectiveswitch A, switch 521, and protection lambda drop, such as protectionlambda drop filter 545, to the respective PIOB, such as PIOB 533.

In step 744, the failure detection node, node 509 and PIOB 533, verifiesthe presence of an “acceptable” signal on the protection path from thefailed node 507 and notifies respective control circuits, circuits509-1. In step 746, the respective control circuits disable WIOB 531,change the state of Switch B, switch 547, and place the respectivetransponder, PIOB 533, into service.

When PIOB 533 is placed into service, it transmits a signal on theprotection channel switch B, switch 547, and respective multiplexer,such as 525, via fibers 510 a, b to the failed node, node 507. In step750, the protection channel lambda, is coupled via switch A, switch 513and protection lambda drop 537, to the respective PIOB, PIOB 529, thuscompleting the process of placing the protection ring into service tocarry traffic between nodes 507, 509. As noted above, variations of theprocess of FIGS. 7A, B come within the spirit and scope of the presentinvention.

FIG. 8 illustrates an embodiment of a portion of a network 200 forproviding protection for a cut fiber. The network 200 includes nodes507′ and 509′ which correspond structurally to and functionally to nodes507, 509 previously discussed.

To provide cable cut protection; fiber 510 can be coupled to splitter204. The split optical signal can be coupled to a working fiber 510-1and a protection fiber 510-2. Splitter 204 would normally be at or nearnode 507′.

At the receiving end, at or near node 509′, a switch 206 and anassociated controller select, for example working fiber 510-1 duringnormal operation. Signals from that fiber are coupled to the opticalinput for node 509′ via fiber 510-3. If the controller for switch 206detects a loss of all signals from fiber 510-1 it changes the state ofswitch 206 and feeds signals from protection filer 510-2 to node 509′.

FIG. 5A illustrates an alternate embodiment to the add/drop multiplexersof FIG. 5. Elements of FIG. 5A which are the same as the correspondingelements of FIG. 5 have been identified using the same numerals.

In FIG. 5A, the protection lambda drops 537, 545 (of FIG. 5) have beendeleted. The traffic lambda drops 511′, 519′ now always drop theprotection lambda. The dropped protection lambda is directed, via SwitchA′, 513′, 521′ to combiner 515, 523 unless there is a failure at node507′.

In the event of a failure at node 507′, the nodes 507′, 509′ carry out afailure recovery process as in FIGS. 7A, B. As part of this process,Switches A′ 513′ and 521′ change state and the dropped lambda is fed toPIOB 529 or 533. The add/drop multiplexers of FIG. 5A should provide amore cost effective implementation than the configuration of FIG. 5. Itwill be understood that other variations are possible and come withinthe spirit and scope of the invention.

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only, to describe the best mode of practicing the invention, andnot limitation. Accordingly, the breadth and scope of the presentinvention should not be limited to any of the above-described exemplaryembodiments, but should be defined only in accordance with the followingclaims and their equivalents.

1. A multi-node optical network comprising: a plurality of spaced apartnodes; a plurality of working communication paths, the members of theplurality extend between and carry traffic in a selected directionbetween selected nodes; a normally unused predetermined protectioncommunication path that extends between all of the nodes forming aclosed ring, the protection path is available to carry traffic in theselected direction, as needed at least between pairs of nodes inresponse to a failure of any one of the working paths between arespective pair of nodes; substantially identical activation circuitryat each node, the activation circuitry at each node of a selected pairof nodes detects a signal loss indicative of the failure of one of theworking paths therebetween, each activation circuitry communicates withthe other via the protection path to place that portion of theprotection path between the selected pair of nodes into service inresponse to failure of any one of the working paths therebetween wherethe activation circuitry at a receiving node of the selected pair ofnodes, in response to a loss of light on a working path from thetransmitting node of the selected pair of nodes, terminates transmissionon the respective working path and initiates a first datalesstransmission via the protection path to the transmitting node, where theactivation circuitry at the transmitting node, in response to a loss oflight on the working path from the receiving node, terminatestransmission on the respective working path and initiates a seconddataless transmission, via the protection path, to the receiving node,the activation circuitry switches at least the second datalesstransmission to traffic carrying between the nodes, and where theremaining members of the plurality of working paths continue to carrytraffic in the selected direction.
 2. A network as in claim 1 where theactivation circuitry at each node responds to both a loss of light onthe respective traffic carrying path and the presence of a receivednon-traffic carrying signal on the protection path.
 3. A network as inclaim 1 where the activation circuitry at each node includes an opticalelement for separating protection path signals from working pathsignals.
 4. A network as in claim 3 which includes a plurality of lightemitting elements associated with respective working paths and aprotection path light emitting element for transmitting traffic on theprotection path, in the event of a failure of one member of theplurality of light emitting elements.
 5. A network as in claim 3 whichincludes an optical switch with an input coupled to an output of theprotection path light emitting element, the switch has a control inputto switch a protection path signal from within the activation circuitryto the protection path for transmission to a receiving node.
 6. Anetwork as in claim 5 where the activation circuitry at each nodeincludes control circuitry to initially transmit as the protection pathsignal a dataless signal on the protection path and to subsequentlytransmit a traffic carrying signal on that path.
 7. A network as inclaim 6 where the control circuitry responds to receipt of a non-trafficcarrying signal on the protection path from another node to transmit atraffic carrying signal on the protection path.
 8. A network as in claim7 which includes a traffic add/drop switch which couples incomingtraffic to one of the members of the plurality of light emittingelements or to the light emitting element of the protection path in theevent of a failure of one of the working paths.
 9. A multi-node opticalnetwork comprising: a normally inactive, singular, looped protectioncommunication path which includes a plurality of network nodes; at leastone working communication path for communicating traffic in a firstdirection between receiving and transmitting nodes; substantiallyidentical receiving and transmitting modules located in respectivereceiving and transmitting nodes of the network with each moduleincorporating first and second monochromatic sources of light, onesource is associated with the working path, the other is associated withthe protection path, and at least one detector of monochromatic lightreceived from the working path; control circuitry in each module coupledto the respective detector and sources, the control circuitry at thereceiving node is responsive to a loss detected thereat, ofmonochromatic light on at least a portion of the working communicationpath where the light originated at the transmitting node, and thecontrol circuitry at the receiving node in response to locally detectedloss of light initiates a multi-step switching process to transfertraffic on at least a portion of the working communication path betweennodes to at least a portion of the protection path between the nodes,the control circuitry at the receiving node transmitting a selecteddataless control signal via the looped protection path to thetransmitting node, and the control circuitry at the transmitting nodesubsequently transferring traffic from the failed portion of the path,between the receiving and transmitting nodes to the portion of theprotection path therebetween for communicating the traffic from thefailed path via the protection path in the first direction, where thecontrol circuitry at the transmitting node responsive, to at least inpart, to a loss detected thereat, of monochromatic light on anotherportion of the working communication path, where the light originated atthe receiving node, transmits a second selected dataless control signalto the receiving node via that portion of the protection paththerebetween with at least the control circuitry at the transmittingnode switching the dataless signal on the protection path between thereceiving and transmitting nodes to a traffic carrying signal.
 10. Amulti-node optical network comprising: a normally inactive, singular,looped protection communication path which includes a plurality ofnetwork nodes; at least one working communication path for communicatingtraffic in a first direction between first and second nodes; first andsecond substantially identical modules located in respective first andsecond nodes of the network with each module incorporating first andsecond monochromatic sources of light, one source is associated with theworking path, the other is associated with the protection path, and atleast one detector of monochromatic light received from the workingpath; control circuitry in each module coupled to the respectivedetector and sources, the control circuitry at the first node isresponsive to a detected loss of monochromatic light on at least aportion of the working communication path where the light originated atthe second node, and the control circuitry at the first node in responseto the detected loss of light initiates a multi-step switching processto transfer traffic on at least a portion of the working communicationpath between nodes to at least a portion of the protection path betweenthe nodes, the control circuitry at the first node transmitting aselected dataless control signal via the looped protection path to thesecond node, and the control circuitry at the second node subsequentlytransferring traffic from the failed portion of the path, between thefirst and second nodes to the portion of the protection paththerebetween for communicating the traffic from the failed path via theprotection path in the first direction, where the modules each includefirst and second optical switches, the first switch couples theprotection path signal to the control circuits, the second switch has aninput coupled to an output from the source of light associated with theprotection path for switching the protection path signal from aprotcction path non-transmitting state to a protection path transmittingstate, both switches have control inputs coupled to the controlcircuits, the control circuits being responsive to a received datalesscontrol signal to switch the signal being transmitted on the protectionpath to a traffic carrying signal.